Gradient-structured allyl cellulose ball arrays for high performance piezoresistive sensors and intelligent handwriting recognition.

Developing flexible pressure sensors that combine ultralow detection limits, high sensitivity, and environmental sustainability remains challenging, particularly using bio-based alternatives to petroleum-derived polymers. We present ultra-sensitive pressure sensors based on multi-scale allyl cellulose ball (ACB) arrays. ACBs were fabricated through syringe-based droplet formation in a saturated NaCl bath and UV-mediated crosslinking of allyl cellulose (AC), yielding a dual-network architecture comprising physically entangled chains from salt-induced phase separation and covalently crosslinked allyl groups that imparts mechanical robustness with controlled diameters (1.2, 1.5, and 2.2 mm). The sensor features a gradient architecture where different-sized ACBs create sequential activation mechanisms: larger balls deform first under low pressure, followed by progressive engagement of smaller balls, achieving ultrahigh sensitivity (27.2% kPa-1), ultralow detection limit (8 Pa), rapid response/recovery (38/40 ms), broad detection range (8 Pa-10 kPa), and exceptional stability (less than 5% drift over 6500 cycles). The approach eliminates expensive lithographic fabrication while surpassing human tactile thresholds. We demonstrated practical applications including flexible electroluminescent devices with programmable patterns and an intelligent handwriting recognition system capable of real-time character classification with 99.3% accuracy. This work establishes a gradient architecture strategy for biodegradable cellulose-based flexible sensors, offering a scalable and cost-effective pathway toward next-generation sustainable wearable electronics and intelligent human-machine interfaces.